Thermal transfer process, electrophotographic process, ink jet process, etc. are currently being investigated as techniques for obtaining color hard copies. The thermal process has advantages in that the apparatuses can easily be operated and maintained, and the apparatuses and materials for consumption are inexpensive. Accordingly, the thermal transfer process has many merits in comparison with other processes.
The thermal process can be roughly classified into two systems. In one system, a thermal transfer dye providing material comprising a base film having a heat-fusible ink layer formed thereon is heated by a thermal head to fuse said ink layer, thus making recording on a thermal transfer image receiving material. In the other system, a thermal transfer dye-providing material comprising a base film having a coloring material layer containing a thermomobile dye formed thereon is heated by a thermal head to allow the dye to thermally migrate to a thermal transfer image receiving material, thus transferring the dye to an image receiving material. In the latter thermal transfer system, the amount of the dye to be transferred can be controlled by changing the amount of energy applied to the thermal head so that gradation recording can be easily made. Hence, the latter system is particularly advantageous for carrying out high-quality full color recording.
However, thermal transfer dye providing materials used in the thermal migration type thermal recording methods have the following disadvantages.
Generally, polymers used in the thermomobile dye providing layer are only soluble in organic solvents, and hence coating solutions for the dye providing layer are dye solutions in organic solvents such as toluene. Further, apparatuses and vessels used in the preparation of the coating solutions must be cleaned with organic solvents. Hence, the apparatuses used for the preparation of coating solutions and coating apparatuses must be equipped with explosion-proof provisions. Moreover, the organic solvents are very expensive in comparison with water and hence manufacturing costs are increased. In addition, problems can arise with the health of the operators of this equipment.
JP-A-60-190389 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method wherein an aqueous ink dispersion containing a nonionic sublimable dye and a water-soluble or water-dispersed resin finely dispersed therein is coated to solve the above-mentioned problems.
In the dye providing layers prepared by this method, however, the dye exists as fine solid particles in water-soluble binders, and hence the transfer efficiency thereof is low. Therefore, it is difficult to obtain a transferred image having sufficiently high density and raw preservability. Further, there is a problem in that heat fusion can occur when water soluble saturated polyester resins are used as the water-soluble binders as described in the aforesaid JP-A-60-190389.
In the preparation of dye providing layers by using conventional organic solvent type coating solutions, the following methods have been proposed to obtain high transfer density and high sensitivity in the thermal migration type thermal transfer recording method. However, these methods have the following problems.
(1) A method wherein the concentration of the dye to the binder in the dye providing layer of the dye providing material is increased.
This method has problems in that when the dye providing material is stored over a long period of time, the dye migrates to the back side of the support or the dye and is precipitated out on the surface of the material, which causes a lowering or unevenness in the concentration of the dye.
(2) A method wherein plasticizers, such as oil in which the dye is soluble, are contained in the dye providing layer to increase the concentration of the dye.
In this method, the dyes can be prevented to some degree from being precipitated out during the storage of the dye providing material, but blocking trouble and the offset of the dye (the migration of the dye to the back side of the support) are likely to be caused. Further, since the coated film is softened, the coated film can be marred, and the heat fusion of the image receiving material to the dye providing material may occur.
Further, repeatedly transferable thermal transfer dye providing materials for use as the dye providing materials used in the aforesaid thermal migration type thermal transfer process have been proposed in JP-A-1-110194, JP-A-1-263084 and JP-A-1-272488. In the repeatedly transferable thermal transfer dye providing materials, the ink layer on the support is composed of a two or more layer structure, in which the different layers provide separate funtions, consisting of a relatively thin surface layer capable of conducting thermal transfer (i.e. an ink layer that contributes to thermal transfer) and a layer having a function capable of diffusing and feeding the dye (i.e. an ink layer as a dye supply layer).
However, the thermal transfer dye providing materials having the plural ink layer structure used in the thermal migration type thermal transfer recording method have the following problems.
It is preferred that the dye concentration in the ink layer as the dye supply layer (hereinafter referred to as (b) layer) is higher than that in the ink layer as the layer contributing to thermal transfer (hereinafter referred to as (a) layer) so that each of the functions of the two or more separate ink layers is effectively exhibited. The higher the dye concentration of the (b) layer, the more preferable. This is because it is preferred that the dye concentration of the (a) layer is kept constant to provide a constant transfer density after transfer is repeatedly conducted and the dye of the (a) layer which is consumed by transfer must be supplemented from the (b) layer.
A problem arises in that when the coating solutions for forming said ink layer are organic solvent systems, the dye is likely to diffuse between the ink layers having different dye concentrations in the lamination of the ink layers, and hence there is a difficulty in providing a sufficient difference in the dye concentration between the layers. Accordingly, there is a disadvantage that the transfer density is lowered by repeated transfer.